This application claims the benefit of Korean Patent Application No. 10-2004-0024106, filed on Apr. 8, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a polyethylene copolymer to be used for water supply pipes, prepared using a supported hybrid metallocene catalyst that can synthesize polyolefin whose physical properties and molecular weight distribution can be more easily controlled compared to a conventional Ziegler-Natta catalyst.
2. Description of the Related Art
A plastic water supply pipe is composed of polyethylene, polyvinyl chloride, polypropylene, polybutene, and the like. Such a plastic pipe has a lower rigidity than a steel pipe, a cast iron pipe or copper pipe, but a demand therefor is increasing due to its high toughness, ease of installation, and superior chemical resistance such as chlorine. In particular, a polyethylene pipe has a higher toughness than a polyvinyl chloride or polypropylene pipe and can be heat bonded, and thus can be easily installed. The polyethylene pipe has also a high resistance to chlorine, which is contained in drinking water, when being used as a water supply pipe. Thus, a demand for the polyethylene pipe is increasing.
However, a conventional polyethylene pipe has been modified by a chemical-crosslinking or moisture-crosslinking due to inferior internal pressure resistance and environmental stress cracking resistance (ESCR) of a polyethylene resin. Such an improvement in the physical properties of the polyethylene resin by the chemical-crosslinking or moisture-crosslinking and an example of applying it to water supply pipes are known in the art.
A chemically-crosslinked pipe is fabricated by extruding a resin composition including polyethylene and an organic peroxide such as dicumyl peroxide in a pipe shape while heating the resin composition to a pyrolysis temperature of the organic peroxide or higher. The organic peroxide is pyrolyzed into organic radicals. The organic radicals generate polyethylene radicals so as to lead to crosslinking of the polyethylene.
A moisture-crosslinked pipe is fabricated by compounding polyethylene, a silane compound such as vinylethoxy silane, an organic peroxide, and silanol condensation catalyst, and then extruding the resulting composition in a pipe shape while heating the composition. A silane crosslinking is performed by exposing the molded pipe to moisture. This technology is disclosed in Japanese Patent Publication Nos. Sho 63-058090 and Japanese Patent Laid-Open Publication Nos. Hei 2-253076 and 7-258496.
Japanese Patent Laid-Open Publication No. Hei 8-073670 discloses a crosslinked polyethylene composition including a copolymer of ethylene and butene-1, having a specific melt index, Japanese Patent Laid-Open Publication No. Hei 9-324081 discloses a crosslinked polyethylene pipe fabricated using polyethylene and an specific antioxidant, and discloses a crosslinked pipe fabricated using polyolefin having a number of double bonds. Japanese Patent Publication No. Sho 57-170913 discloses a crosslinked pipe fabricated using polyethylene with a specific density and molecular weight, Japanese Patent Laid-Open Publication Nos. Hei 9-020867 and 7-157568 disclose a crosslinked pipe fabricated using a silane modified graft polyethylene with a narrow molecular weight distribution, and Japanese Patent Laid-Open Publication No. Hei 7-041610 discloses a crosslinked pipe for drinking water, fabricated using a specific organic peroxide. Japanese Patent Publication No. Sho 60-001252 discloses a crosslinked pipe fabricated using an activated carbon, silica, and alumina, Japanese Patent Laid-Open Publication No. Hei 10-182757 discloses a pipe for supplying water or hot water, fabricated using a specific organic unsaturated silane and a specific radical generator, Japanese Patent Laid-Open Publication No. Hei 7-330992 discloses a method of fabricating a pipe using an epoxy compound, and Japanese Patent Laid-Open Publication No. Hei 6-248089 discloses a crosslinked pipe fabricated using a high density polyethylene.
However, the polyethylene resin that is used as a raw material in the conventional technologies is prepared using a conventional polymerization catalyst such as a Ziegler-Natta catalyst or vanadium catalyst. The use of such an ethylene polymer causes various problems. That is, when the conventional ethylene polymer that has a broad molecular weight distribution and more comonomers incorporated in low molecular weight components than in high molecular weight components is used to fabricate a crosslinked pipe, the low molecular weight components are mainly crosslinked and the high molecular weight components are not sufficiently crosslinked. Thus, the crosslinked pipe has an inferior mechanical strength, in particular, internal pressure creep resistance at high temperatures.
When molding a moisture-crosslinked pipe, a large amount of an unsaturated silane compound should be added in order to sufficiently occur a silane crosslinking in the high molecular weight compounds. The silane crosslinked pipe has an offensive odor due to the unsaturated silane compound. In the process of pipe extrusion, a long term processing is difficult due to die gum from polyethylene residue.
A metallocene catalyst system comprises a main catalyst whose main component is a transition metal compound, mainly a Group IV metal and an organometallic compound cocatalyst whose main component is aluminum. Such a catalyst offers a polymer having a narrow molecular weight distribution depending on the single site characteristics. The molecular weight and molecular weight distribution of polyolefin are important factors in determining the fluity and mechanical properties that affect the physical properties and processability of a polymer. In order to manufacture various polyolefin products, it is important to improve melt processability through the control of the molecular weight distribution (C. A. Sperat, W. A. Franta, H. W. Starkweather Jr., J. Am. Chem. Soc., 75, 1953, 6127). Especially for polyethylene, physical properties such as toughness, strength, ESCR, etc. are very important. Therefore, a method of preparing a polyolefin having a bimodal or broad molecular weight distribution in order to enhance mechanical properties in high molecular weight components and processability in low molecular weight components has been proposed.
Recently, attempts to prepare an ethylene polymer with a molecular weight distribution of 2-3 using a catalyst resulted from a metallocene compound and aluminoxane and the like are disclosed in Japanese Patent Publication Nos. Sho 58-019309, 60-035006, 60-035007, 61-130314, 61-221208, 62-121709, and 62-121711. Japanese Patent Laid-Open Publication No. Hei 10-193468 discloses a crosslinked pipe fabricated using a polyethylene obtained by a metallocene catalyst. However, since this ethylene polymer has, in particular, insufficient fluity due to a narrow molecular weight distribution, heat is generated in an extruder and a premature crosslinking partially occurs. Thus, the extruded pipe has a rough surface and a lower mechanical strength.
The chemically crosslinked pipes and moisture-crosslinked pipes are not suitable for drinking water due to the remaining unreacted monomers, flexibility in the installation is diminished, and heat bonding is difficult.
The inventors made efforts to design an polyethylene composition that can maintain rigidity by increasing a comonomer content in high molecular weight components and decreasing a comonomer content in low molecular weight components, in order to fabricate a pipe having a sufficient resistance to stress without crosslinking of polyethylene. As a result, the inventors prepared an ethylene-based copolymer having a bimodal or broad molecular weight distribution and superior processability, resistance to stress, and ESCR due to copolymerization of ethylene and C3-20 α-olefin mainly occurring in high molecular weight chains by using a supported hybrid catalyst in which a metallocene compound suitable to prepare a low molecular weight polyethylene and a metallocene compound suitable to prepare a high molecular weight polyethylene are supported on a support, thereby completing the present invention.